1. Field of the Invention
This invention relates to an exhaust purification device for internal combustion engine, specifically a technique of improving the purification performance of a catalytic converter by forcibly modulating the air/fuel ratio of exhaust.
2. Description of the Related Art
A three-way catalytic converter for exhaust purification using a noble metal such as platinum (Pt) or the like has a considerable capacity to store oxygen (O2). When the air/fuel ratio of exhaust is lean (oxidizing atmosphere), it stores O2 and thereby suppresses the production of NOx, and when the air/fuel ratio of exhaust is rich (reducing atmosphere), it releases the O2 stored and thereby accelerates the oxidation of HC and CO. By this, the exhaust purification performance improves.
Hence, in recent years, vehicles have been developed and put to practical use in which improvement in the exhaust purification performance of the three-way catalytic converter is intended by forcibly modulating the air/fuel ratio of exhaust between a lean air/fuel ratio and a rich air/fuel ratio, for example by switching the air/fuel ratio in the combustion chamber of the internal combustion engine between a lean air/fuel ratio leaner than a specific air/fuel ratio (stoichiometric air/fuel ratio, for example) and a rich air/fuel ratio richer than the specific air/fuel ratio, with a specific period and a specific amplitude.
Further, a device has been developed in which improvement of the forcible modulation control is intended by monitoring the air/fuel ratio of exhaust (referred to as “exhaust air/fuel ratio”) by an exhaust sensor during the forcible modulation and performing feedback control so that the actual exhaust air/fuel ratio agrees with a target exhaust air/fuel ratio (see Japanese Unexamined Patent Publication No. hei 10-131790).
As exhaust sensors for detecting the exhaust air/fuel ratio, a wide-range air/fuel sensor (linear air/fuel ratio sensor (LAFS), for example) and an oxygen sensor (O2 sensor, for example) are known. However, as disclosed in the above-mentioned Patent Document, in order to perform feedback control so that the actual exhaust air/fuel ratio agrees with a target exhaust air/fuel ratio, it is necessary to detect the exhaust air/fuel ratio over a wide range, accurately. Hence, in general, the wide-range air/fuel sensor is used to detect the actual exhaust air/fuel ratio.
However, while the wide-range air/fuel sensor can detect a wide range of air/fuel ratios, it has a drawback that its cost is very high. Hence it is not practical.
Meanwhile, the oxygen sensor is low in cost and therefore very advantageous for general frequent use. However, it has a non-linear output characteristic curve with respect to air/fuel ratio, so that the range of detectable air/fuel ratios is narrow. Hence, there is a problem such that, when the amplitude of the forcible modulation is increased to improve the exhaust purification performance, the exhaust air/fuel ratio exceeds the range of air/fuel ratios detectable by the oxygen sensor, so that the exhaust air/fuel ratio cannot be detected accurately on the basis of the output from the oxygen sensor.